Cathode for electronic tubes



Aug. 6, 1940. D. D. KNOWLES CATHODE FOR ELECTRONIC TUBES Filed Sept. 16,1937 WITNESSES:

1 INVENTOR Dewey 0. Know/es.

Patented Aug. 6, 1940 UNITED srAr-ss 2,210,674. CATHODE ELECTRONIC TUBESDewey D. Knowles,,Wilkinsburg, Pa yassignor to Westinghouse Electric &Manufacturing Company; East Pittsburgh, Pa., a corporation ofPennsylvania Application September 16, 1937, Serial No. 164,147

' Claims. (01. 25.0-27.5)

My invention relates to electrical discharge devices, and in particularto electron-emissive electrodes for such devices. It is particularlyadapted to devices of the above mentioned'type 5 which embody a controlelectrode for regulating the discharge passing from theelectron-emissive electrode, and especially to such devices as containagaseous atmosphere which plays a substantial part in the transport ofelectric current therein.

= In electrical discharge devices of the type in which at least oneelectrode is artificially heated to a temperature at which its surfacereadily emits electrons, considerable difliculty has been found inproviding a type of electrode which will not deteriorate substantiallyin the course of protracted operation. This. is particularly true wherea gaseous atmosphere of substantial pressure is present in the tube. Acommon type of electron-emissive cathode for high-vacuum tubes is oneinwhicha metallic core adapted to be electrically heated is coated withoxides of barium and/or strontium which readily emit electrons uponbeing heated to a temperature of the order of 900 C. It has been found,particularly where a-substantial gaseous atmosphere is present, that theoxide coatings tend to break or, possibly because of the severebombardment of gaseous ions which the electrical discharge subjects themto. Thetendency of the discharge, where substantial pressures of gas arepresent, to concentrate in a cathode spot of restricted area probablyplays a part in this disintegrating tendency also. a 1 One object of myinvention is accordingly to disclose several ways of constructing thermionically-emissive cathodes which I have found to be great improvementsover the above described oxide-coated cathodes and other cathodes of theprior art with which I am acquainted.

Another object of my invention is to disclose several types of cathodeswhich make it possible to maintain a considerable gaseous atmosphere inthe immediate vicinity of the electron-emissive surface, but at the sametime, to maintain amuch smaller pressure of gas in certain otherportions of the path of the discharge emanating at the said cathode. T

I Another object of my invention is to disclose an electrical dischargetube in which the control electrode is particularly effective inregulating the discharge from the cathode to any cooperative'electrode.1 i i I Still another object of my invention is to disclose anelectricaldischarge tube in which the voltage drop through the tubecharacterizinga given ourrent between .the electrodes is considerablysmaller than that characterizing the flow of currentof'the same.magnitude between electrodes separated .by the same distance, but of thetype characteristic of the prior art.

.Another object of .my invention is .to disclose an electrical dischargetube in which much larger instantaneous currents may flow from thethermionically-emissive electrode without injury to the latter than wasthe case in tubes utilizing thermionically-emissive electrodes of theprior art. v p

Still another object of my invention is to disclose a form ofthermionically-emissive' electrode in which the rate of disappearance ofemissive material from the cathode surface is materially less than thatoccurring in the case of the same material operating at the sametemperature in cathodes of the prior art.

Still another object of' my invention is to "disclose a form of cathodeor electrical discharge devices in which theloss of a given amountofmaterial from the cathode surface will have a.

less injurious effect than wasthe case in-cathodes of the prior-art.

A further object of my invention is to disclose a method of making athermionically-emissive cathode which is convenient, economical, andreadily adapted to quantity production.

With the foregoing objects and principles in mind, my invention willbest be understood by reference to the following description, taken inconjunction with the accompanying drawing in which:

Figure 1 illustrates a longitudinal mid-section of an electricaldischarge tube embodying my invention; s

Fig. 2 illustrates one particular form in which a cathode embodying theprinciples of my invention may be constructed, and

Fig. 3 is a drawing useful in describing the method of constructing thecathode shown in Fig. 2.

Referring particularly to Figure 1, a vacuum type container I, which maybe of glass or other suitable material, has supported in its interior inways well known in the art, an anode 2 and a control electrode 3, bothof which may be of conventional form; An electrode adapted to emitelectrons thermionically is shown at land comprises a heater 5 suppliedwith electric current through leads 6 and a crater-likeelectron-emitting element 1 of a type more fully illustrated in Fig. 2.The heater 5 may, if desired, be embedded in electrically non-conductingrefractory coating 8, and

the latter may be surrounded by metallic sleeves 9 and II, thelattersupporting the cathode from the envelope I. For many purposes it will bedesirable to provide a quantity of a material I2, such as mercury insuflioient quantity so that it reaches just to, or a little above, thelower face of the body 7. The material I2 may, if desired, constitutethe connection from the lower end of heater 5 to the lower lead 6.

Referring particularly to Figs. 2 and 3, the member I is formed bywinding up into a spiral a tapered metallic ribbon coated withelectronemissive material, such as barium-oxide and strontium oxide. Asillustrated in Fig. 3, the narrow end of the ribbon may be attached to amandrel I3 and the ribbon thereby wound, by revolving the latter, toform a spiral structure such as appears in Fig. 2. It will be desirablefor some purposes to leave the mandrel I3 in place to form a lead bywhich current may be introduced into the innerend of the spiral, thecur.- rent being taken out again through a second lead attached to theouter. :end of the ribbon. Electron-emissive coatings which are fairelectrical insulators are well known, and when these are used thecurrent flows along the ribbon of the spiral from itsinner toits outerend. It may be desirable in certain instances to let the turns of thespiral spring slightly apart after winding it .so. that spacings existbetween the successive turns when the spiral ,is in position in thedischarge tube. Such spacing might even be insured by superposing aspacing-layer of metal or some suitable othermaterial upon the ribbon atthe time it is wound. into the spiral, this auxiliary spacer beingremoved after the winding operation.

For many purposes, however, it will be desirable to remove the mandrelI3 after the completion of the winding operation, and in the cathodeillustrated in Fig. 1 the mandrel has been .removed. The spiral is thenpositioned by suitably supporting its outer turn from the tube wall, forexample, by the in-leading wire 6.

Instead of forming the electroneemissive material I from an oxide coatedribbon as described above,it maybe formed by molding a mixture of nickelor other metallic particles and electronemissive material suchas amixture of barium oxide and strontium oxide, said mixture being moldedin the form of a cylinder with a depressed ,crater in its center in muchthe same shape as the crosssectioned portion ofFig. 2. Pearcy Patent1,981,245 described in detail the method of molding such a mixture.

Upon the connection of such a tube as is shown in Fig. 1 into aconventional radio circuit, the flow of heating current-through heater 5will cause the surfaces of the emissive oxides to freely emit electrons,and current flow will start between the anode 2 and cathode 4. Thegaseous atmosphere, particularly in the interior of the crater ofcathode 1, and in any spaces which may exist between successive turns ofthe spiral will become readily ionized. The same is true of the oxideparticles and small cavities between the particles of the sinteredmatter disclosed in the Pearcy patent should the latter be employedinstead of the ribbon spiral of Fig. 2.

V The external coating ofthe cylinder I I may be made of ,a good heatreflecting. material or some other structure ofiering resistance to theoutflow of heat may be employed to minimize flow of heat from thecathode4 to its surroundings.

which only one of the Where a readily vaporizable material such asmercury is made to contact thelower face of the cathode 4, it will befreely vaporized, particularly in the interior of the element II andprovide a gaseous atmosphere of substantial pressure within andimmediately above the crater. On the other hand, it is possible to sodesign the tube I by principles well known in the art in such a way thatthe'vapor will be condensed not far above the cathode 4, and a continualup-flow of vapor through the crater and condensation and down-flowoutside the walls of the cathode 4 will occur. In this case the vaporpressure in the neighborhood of the control electrode 3 may be very muchlower than that within and immediately above the element 1. The same istrue of the vapor pressure between the control-electrode 3 and the anode2. As a result of this pressure distribution, the effectiveness incontrolling the discharge of the grid 3 is greatly increased over whatit would be if the dense vapor like that in the crater I. filled theentire interior of the tube I.

As a specific example, the tube illustrated in Fig. 1 might have anoverall length of 15 centimeters, a diameter of 8 centimeters and bedimensioned to operate on a 440 volt supply delivering 6 amps. steadyload current. The grid-tocathode voltage should then be of the order of6 volts. The width of the ribbon shown in Fig. 3, which might be ofKonal, nickel or the'like, might at its two ends be centimeter and 1centimeters respectively; its thickness be .01 centimeter; the centralhole be centimeter in diameter, and the outside diameter of the spiralbe 1 centimeter. The heater 5 might take 11 amps. at 5 volts; therefractory wall 8 be centimeter thick by 1 centimeters high; and thesleeves 9. and ll be of nickel .01 centimeter thick by 2 centimetershigh. The control electrode 3 might be of carbonized nickel mesh wires.02 centimeter in diameter and be spaced 2 centimetersfrom the top ofcathode 4 and 2 centimeters from the face of anode 2. 4

Were the member 1 made of the sintered mixture mentioned above, itsoverall dimensions might be the same asthose given above for the spiraltype. 1

It is possible, insteadof employing the vaporizable liquid I2, to fillthe container I with a permanent gas at a desired pressure, and even insuch a gas the crater-like cathode herein described has many advantages.It is also possible to so proportion the amount of the liquid I2 thatits upper surface is separated by some distance from thelower face ofthe cathode 4,,thereby regu lating therelative magnitudes of thevaporization occurring immediately below the center of the crater 1 andthat occurring entirely outside the confines of the cathode 4. It iswithin the purview of my invention to provide fluid cooled surfaces inthe region external to and above the periphery of the cathode 4 toassist in regulating the rapidity of condensation of vapor emanatingfrom the crater I. I

While I have described a discharge device in principal electrodes is ofthe thermionically-emissive type, application of the construction whichall of the electrodes or all of the electrodes except those performing acontrol function, are of this type is within the scope of my invention.The typeof. cathode which I have'describ'ed is particularly useful intubes such as I have'described which contain a gaseous 1 atmosphere,

above described to tubes in' but employment of cathode structures of thesame type in high-vacuum tubes will still be found to possess certainadvantages over the prior art.

By reason of the fact that a large proportion of thesurface of the oxidecoatings are faced by surfaces similarly coated with oxide, materialvaporized at one point may be projected and condensed upon a similarsurface at another point. In consequence of this, the total loss ofoxide from the cathode crater goes on only very slowly.

Likewise the intense electronic bombardment of I the gaseous atmospherewithin the crater results,

in an extremely low cathode-voltage drop and low electrical losses.

Where the liquid 12 is an electrical conductor, as in the case ofmercury, a cathode spot may form on the mercury surface itself,particularly when the load current is high. As a result of this thestructure which I have described has instantaneous and othercurrent-overload capacities far in excess of arrangements known to theprior art.

In accordance with the patent statutes, I have described one particularembodiment of my invention, but the principles thereof are capable ofquite different applications in embodiments which will be apparent tothose skilled in the art. I accordingly desire that the following claimsbe given the broadest construction of which their language is reasonablycapable.

I claim as my invention:

1. In combination with a vacuum-tight container enclosing a firstmain-electrode and control-electrode together with a material whichvaporizes readily not'far above room temperature, a secondmain-electrode comprising a heater and a core comprising a spiral oftapered ribbon in contact with said material.

2. In combination with a vacuum-tight container enclosing a firstmain-electrode and control-electrode together with a material whichvaporizes readily not far above room temperature, a secondmain-electrode comprising a heater and a core comprising a spiral oftapered ribbon.

3. An electrode adapted to emit electrons freely when heated comprisinga spiral of tapered ribbon.

4. An electrode adapted to emit electrons freely when heated comprisinga spiral of tapered ribbon coated with thermionically-emissive material.

5. In combination with a vacuum-tight container enclosing the first mainelectrode and a control-electrode together with a material'whichvaporizes readily not far above room temperature, a secondmain-electrode comprising a heater and a core which comprises a spiralof tapered ribbon coated with thermionically-emissive .material, saidspiral beingin good heat exchanging relation with the first saidmaterial.

DEWEY. D. KNOWLES.

